1. Introduction

AD7237A is a LC2MOS dual 12-bit digital-to-analog converter launched by AD Company of the United States. It has the characteristics of high speed, low power consumption, wide operating voltage, etc., and has been widely used in industry. This paper briefly introduces the basic structure and pin functions of AD7237A. Then, it elaborates on its application method in the design of computer interface expansion card.

2. Basic structure and pin functions of AD7237A

AD7237A is a complete dual 12-bit voltage output digital-to-analog converter with output amplifier and built-in reference voltage source. And it has the following main features:

● High speed: typical data settling time is 30ns;

● Low power consumption: in the case of unipolar output, the typical power consumption is 165mW;

● Operating voltage: 12~15V;

● (8+4) bit data latch structure.

AD7237A is an improved version of industrial AD7237. The main difference between AD7237A and AD7237 is that AD7237A has a wide operating voltage range (12V~15V), fast conversion speed, and good anti-VDD voltage interference performance (VDD fluctuation range is ±10%).

Figure 1 is a basic structural block diagram of AD7237A. AD7237A mainly includes four parts: reference voltage source, control logic circuit, A-channel digital-to-analog converter and B-channel digital-to-analog converter. The chip integrates a reference voltage source for users to use. When the working power supply voltage reaches 12V, even if the working power supply voltage fluctuates by ±10%, the standard 5V power supply is still output at the second pin REF OUT. The control logic circuit has 5 control signals: CS, WR, A1, A0, LDAC, which are used to control AD7237A to complete various functions. The A-channel digital-to-analog converter consists of a reference voltage input circuit, a two-stage data latch, a resistor network and an output amplifier. The B-channel digital-to-analog converter is exactly the same as the A-channel.



The pin functions of AD7237A are listed in Table 1.



3. Application of AD7237A in Computer Expansion Card Design

Due to the characteristics of AD7237A such as fast conversion speed (30ns), low power consumption, high precision, and dual-channel, it is widely used in industrial design. According to these characteristics of AD7237A and other specific requirements, the author of this article tried to apply it to the computer expansion card circuit and achieved good results. Figure 2 is the circuit diagram of the digital-to-analog conversion part of the interface expansion card, which is inserted into the ISA slot (also called AT slot).



In Figure 2, the chip select signal CS and data storage signal LDAC of AD7237A come from the output pins F0 and F1 of GAL16V8. Since only the addresses 0X0000-0X03FF are used for the I/O port on the computer motherboard, only A2-A9 need to be input into GAL16V8, and A0 and A1 need to be directly input into AD7237A. Other input signals of AD7237A, such as data lines DB0-DB7, write signals WR, power supplies VDD, VSS, ground lines DGND and AGND, all come from the computer motherboard. Because both A and B are required to be bipolar outputs, pins REF INA, ROFSA, REF INB, and ROFSB are connected to pin REF OUT. Then, VOUTA and VOUTB are sent to two operational amplifiers respectively to double the voltage to -10V~+10V. The follower in the figure is designed to increase the output impedance. The

normal operation of the circuit depends on the correct working timing. Figure 3 is the working timing diagram of GAL16V8. When A2~A9 are valid, G0 and G1 are output with a delay of td (3ns
● The width of the write pulse TWR ≥ 100ns;

● The time from data valid to WR signal establishment TDSET> 80ns;

● The time from data valid to WR signal holding TDHOLD> 10ns;

● The width of the latch LDAC pulse TLOAD> 100ns.





In Figure 4, 16ns < t2 < 128ns, t4 > 10ns, 10ns < t5 < 35ns, 10ns < t6 < 35ns, t7 > 360ns, 3ns < t8 < 25ns, so all the above timing requirements can be met. The

computer interface expansion card can be used in the coordinated control system of two AC motors. The control system consists of a PC (PC-486 or PC-586), two sets of AC servo systems, two AC motors and two electric encoders, as shown in Figure 5. The computer detects the signal of the photoelectric encoder through the interface card, and then controls the AC servo system to drive the servo motor through the interface card. The PC can implement various control algorithms to make the two AC motors run in coordination. The role of AD7237A in this system is to receive two digital quantities from the computer, convert them into analog quantities, and send them to the analog interfaces of the two sets of AC servo systems respectively.